1. Field of the Invention
The present invention relates to the field of turbine engines, and in particular, turbine engines in which distinct engine components are integrally cast so as to increase resistance to thermal stress, and thus decrease the need for cooling flow across the engine components.
2. Description of the Prior Art
Turbine engines, and in particular, gas turbine engines, are typically deployed in aircraft for jet propulsion. A gas turbine engine may form the core of a conventional turbojet engine, or may be used in conventional hybrid applications, such as in turboprop engines combining both jet and propeller propulsion.
A problem that occurs in gas turbine engines is that the components of the engine often cannot withstand prolonged and repeated exposure to the hot gases created during the combustion process. Engine components, such as combustion exit ducts, are typically formed of thin sheets of wrought metal which cannot withstand the prolonged, cyclic exposure to hot engine gases which are typically created during engine operation. As a result, these components require some mechanism for cooling in order to maintain their structural integrity over the service life of the engine.
In recent years, solutions have evolved for cooling engine combustion chamber components during operation of the engine. Proposed solutions are set forth in U.S. Pat. No. 5,271,220 to Holmes et al, issued Dec. 21, 1993, and U.S. Pat. No. 5,280,703 to Corrado et al, issued Jan. 25, 1994. These patents disclose radial inflow gas turbine engines in which the combustion chamber is surrounded by a case wall. Relatively cool airflow from the compressor stage of the engine is vented between the case wall and the outer wall of the combustion chamber to cool the combustion chamber walls during engine operation. The cooling airflow extends around the combustion chamber walls. The cooling airflow is ultimately vented into the combustion chamber where it is released in the exhaust stream.
A third construction is disclosed in U.S. Pat. No. 4,439,982 to Weller et al, issued Apr. 3, 1994. In this invention, airflow is directed around the combustion chamber during which time it is drawn on for use in the combustion process. The combustion products are then directed to a reverse flow duct where they flow across alternating rows of stator vanes and turbine blades. The airflow directed around the combustion chamber is vented through a duct into the interior of the stator vanes for cooling the stator vanes during engine operation. The airflow entering the interior of the stator vanes is then released into the exhaust stream through holes near the trailing edge of the stator vanes.
Both Holmes et al and Corrado et al relate to radial turbines and are not really pertinent. Each of the above-mentioned references provides for airflow which cools the walls of the combustion chamber and, in the case of Corrado et al, cools the turbine nozzle as well. The high volumes of cooling flow necessary to cool these components essentially deteriorates the efficiency of the engine. This occurs because a significant portion of the airflow being directed towards the combustion chamber by the compressor stage is not used by the combustion chamber. Instead, a significant portion of this airflow is used for transferring heat away from the combustion chamber and associated components. This means that more of the airflow from the compressor stages must be used for cooling and thus less of the airflow may be used for combustion.